Los chicos todo lo que vieron.
han decido
han dicido
han dicho
han diciendo

The correct answer is han dicho.

Explanation:

In this case we are faced with verb tense pretérito perfecto.

We use it to express actions that occurred in the past and extend to the present, and actions that happened in the past and that have as a consequence a situation or result related to the present or the future.

To conjugate a verb in the past perfect we use the auxiliary haber in present and the participle of the main verb.

Yo he dicho

Tú has dicho

Él/Ella/Usted ha dicho

Nosotros/as hemos dicho

Vosotros/as habéis dicho

Ellos/Ellas/Ustedes han dicho

answer might be "si", would be better if you added answer choices

b.

explanation:

just because.

well, the b sentence is the only one that has sense.